Metallic nanowire and method of making the same

ABSTRACT

A metallic nanowire having an aspect ratio of at least 100 and a diameter less than 200 nanometers composed of at least one of bismuth, indium, tin, lead, zinc, antimony and alloys of the same and a method of making the same from a thin film composite.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention pertains to metallic nanowire structures andmethods of making the same.

[0003] 2. Brief Description of Relevant Art

[0004] Nanowire structures have utility in various electronic andmicroelectronic applications. The unique electrical, optical, magnetic,catalytic, mechanical and tribological properties of such nanostructuredmaterials provide numerous opportunities in a variety of applications.Various metallic compositions have been considered for integration intonanowire structures. Of particular interest are materials havingcharacteristic properties which differ from various common metals in atleast one aspect such as anisotropic Fermi surface, low carrier density,small carrier effective mass and long carrier mean free paths.Additional advantageous characteristics can include materials whichexhibit very large positive magnetoresistance at room temperature. Suchnanowire structures advantageously include metallic compounds whichcontain bismuth, tin, lead, indium, zinc, or antimony.

[0005] Heretofore, various approaches have been employed to producemetallic nanowires. Single filaments of materials such as bismuth withdiameters in the micrometer range have been made using various processessuch as the Taylor process. Nanowires containing materials such asbismuth having diameters as small as 200 plus nanometers have beenfabricated using high pressure casting from a liquid phase.Additionally, porous alumina host media have been employed to producemetallic nanowires containing materials such as bismuth by pressureinjection of molten bismuth or by infiltrating and condensing bismuthvapor into porous alumina channels. The resulting nanowire had adiameter between 10 and 200 nanometers. The resulting nanowires producedby the methods are characterized by multiple crystalline regions andessentially short finite lengths. The lengths of nanowires produced bysuch methods typically is one which provides an aspect ratio of lengthto diameter of 10 or less.

[0006] Thus, it would be desirable to produce a metallic nanowire havingenhanced characteristics such as at least one of enhanced crystallinedomain, elevated aspect ratio, small diameter and increased length. Itis also desirable to provide a method whereby such nanowire material canbe produced efficiently and economically in a highly reproduciblemanner.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a metallic nanowire andmethod of making the same. The metallic nanowire is composed of at leastone of bismuth, indium, tin, lead, zinc, antimony, alloys containingbismuth, alloys containing indium, alloys containing tin, alloyscontaining lead, alloys containing zinc, and alloys containing antimony.The metallic material of the nanowire is present as at least one unitaryelongated crystalline structure having an aspect ratio of at least 100and diameter less than 500 nanometers.

[0008] The method for producing nanowires includes the steps of forminga composite film having at least one outwardly oriented surface. Thecomposite film contains at least two phases and exhibits initial highcompressive stress. The first phase of the composite film is a metallicmaterial which exhibits a characteristic metallic material meltingtemperature. The metallic material of the first phase includes at leastone of bismuth, indium, tin, lead, zinc, antimony, and alloys thereof.The second phase functions as a host matrix and has a meltingtemperature greater than the melting temperature of the metallicmaterial. In the production method of the present invention, the initialhigh elevated compressive stress exhibited in the composite film ispermitted to relax to a subsequent lower elevated compressive stress.The metallic material of the first phase is permitted to diffuse withinthe composite film and to emit from at least one initiation pointlocated on the outwardly oriented surface of the composite film. Theemission event occurs concurrently with the relaxation to the subsequentlower compressive stress value. The emitted metallic material exhibitsan elongated crystalline metallic structure, and an aspect ratio of atleast 100.

DESCRIPTION OF THE DRAWING

[0009]FIG. 1 is a process diagram of the nanowire formation method of anembodiment of the present invention;

[0010]FIG. 2 is a perspective view of delayed release metallic nanowirestructure of an embodiment of the present invention;

[0011]FIG. 3 is a perspective view of metallic nanowire of an embodimentof the present invention;

[0012]FIG. 4A is a low magnification scanning electron microscope imageof nanowires prepared according to a method of the present inventionshowing a panoramic view of the sample surface with bismuth nanowires;

[0013]FIG. 4B is a scanning electron microscope image of a high aspectratio bismuth nanowire on the sample surface of a composite filmprepared according to a method of the present invention;

[0014]FIG. 4C is a detailed SEM image of composite film preparedaccording to a method of the present invention showing an individualnanowire emerging from a pinhole defect initiation point;

[0015]FIGS. 5A and 5B are SEM images of the same area of composite filmprepared according to a method of the present invention prepared as afracture sample with the respective images taken five minutes apart; and

[0016]FIG. 6 is a transmission electron microscope image of severalbismuth nanowires prepared according to a method of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The present invention is predicated, at least in part, on theunexpected discovery that metallic nanowires having extended singlecrystalline regions can be produced from composite film structures. Theresulting nanowires are expected to exhibit enhanced transportproperties which can be advantageously employed in a variety of end-useapplications.

[0018] In the method of the present invention, a composite film isemployed as the platform for preparing and producing nanowires. Thecomposite film may be prepared by any suitable method which provides athin film composed of two distinct phases having an outwardly orientedsurface. The composite film includes a first phase which includes ametallic material. The first phase is surrounded by a second phase whichfunctions as the host matrix. The second phase comprises a materialwhich does not exhibit appreciable chemical or thermodynamic mixing withthe metallic material of the first phase. More specifically, the hostmatrix material is one which cannot form an appreciable alloyed statewith the metallic material of the first phase. Thus, the composite filmemployed in the method of the present invention is one which will remainas a composite exhibiting at least two phases during the nanowireproduction process. The composite film of the process has a host matrixtogether with regions of concentrated or accumulated metallic material.The host matrix is composed of a material which will permit controlleddiffusion and/or accumulation of the metallic material of the firstphase within the matrix. Controlled diffusion and/or accumulation may beaccomplished as a result of any suitable inherent physicalcharacteristic and/or external means. Preferably, controlled diffusionand/or accumulation of the metallic phase is accomplished, at least inpart, by a differential which exists between the melting temperature ofthe first and second phases. Typically, the matrix material will have amelting temperature at least 30% greater than the melting temperature ofthe metallic material which makes up the first phase. The meltingtemperature differential can be greater as desired or required. Thus,the present invention contemplates melting temperature differentials inwhich the melting temperature of the matrix phase is as great as four tofive times the melting temperature of the metallic material.

[0019] It is believed that the nature of the matrix material and itselevated melting temperature with respect to the metallic materialpermits diffusive movement of metallic phase material with respect tothe host matrix. It is believed that the movement of metallic materialfacilitates microscopic extrusion through initiation points of stressconcentration during the compressive stress relaxation phenomena.

[0020] The metallic material employed in the first phase is one whichwill provide desirable electrical, optical, magnetic, catalytic,mechanical and/or tribological properties in a nanostructured material.Such materials include bismuth, tin, lead, indium, and antimony. Thisinvention also contemplates that the metallic material may be alloyscontaining one or more of the aforementioned metals. The materials ofchoice are ones which exhibit diffusion in the host matrix, preferablyat temperatures at or near ambient or room temperature. The metallicmaterials of choice are also ones which demonstrate a suitable surfaceinteraction with the associated matrix material such that a significantdegree of filament formation occurs as metallic material is extrudedfrom stress points in the film surface due to compressive stressinherent in the composite film. By the term “significant degree offilament formation” it is meant that the amount of nanowire formation issuch that the nanowires can be produced and collected in an appropriatemanner. Thus, the surface interaction between the matrix phase materialand the metallic material of the first phase at the emission event issuch that a significant degree of the metallic material emits as ananowire rather than a surface coating. While it is contemplated thatsignificant amounts of the first phase metallic material emit as ananowire, some limited amount of coating phenomena may be toleratedprovided that any coating phenomenon does not appreciably interfere withthe nanowire formation.

[0021] The matrix material may be one which can maintain the desiredmetallic material in a dispersed yet diffusible state. Typically, thematrix material of choice will be one which will form an appropriatecomposite thin film with the metallic material contained therein andwill exhibit an elevated degree of compressive stress upon initial filmformation. As used herein, the term “elevated compressive stress” isdefined as a compressive stress sufficient to initiate and sustainemission of metallic material from the host matrix in the form of ananowire. Typically, initial elevated compressive stress of thecomposite film will be greater than about 200 MPa upon initial filmformation. Greater initial compressive stress is also considered withinthe purview of this invention with compressive stress values between 200MPa and 450 MPa being contemplated and compressive stress values over450 MPa being considered within the purview of this invention. As usedherein, the value of compressive stress is measured using the wafercurvature measurement technique and Stoney's equation as outlined in“Proceedings of the Royal Society, ” Vol. A82, page 172 (1909).

[0022] The host matrix phase can be any suitable material system whichmeets the aforementioned requirements and is essentially nonreactivewith the metallic material employed in the first phase. Typically, suchmaterial systems are nitrides, carbides, oxides, and borides andcarbon-based materials and their combinations being preferred. Thetypical nitride is, preferably, a complex of a material such as chromenitride, aluminum oxides, boron carbides, titanium nitride, diamond-likecarbon materials or the like.

[0023] The thin film from which the nanowire is emitted can be producedby any suitable coating method which will provide compressive stress inthe composite thin film thus produced in the desired ranges. Films canbe prepared on suitable substrates using methods such physical vapordeposition, chemical vapor deposition, plasma spray, thermal spray andthe like.

[0024] The composite film thus formed will typically have a totalthickness in a range sufficient to efficiently permit the emission ofthe metallic material in a nanowire structure. Typical film thicknessesare those less than about 10 micrometer ranges between about 0.5 and 5micrometers being preferred and ranges between about 0.1 and about 2.0micrometers being most preferred.

[0025] Referring to the process diagram of FIG. 1, in the process of thepresent invention, once the composite film has been formed at 110,relaxation of the initial elevated compressive stress is initiated at112 thereby producing or promoting phase diffusion within the filmconcurrent with the relaxation of compressive stress. The diffusion ofthe metallic material of the first phase proceeds at a rate whichgenerally corresponds to the rate of the relaxation compressive stress.Relaxation of compressive stress and diffusion produce emission ofnanofiber material 114 at location(s) in and on the surface of thecomposite film. These locations are initiation points for nanowiregrowth and are areas of stress concentration located in, and moreparticularly on, the surface of the composite film. Locations of stressconcentration(s) can include, but are not limited to, naturallyoccurring and artificial areas of film discontinuity such as naturallyor artificially induced fractures, fault lines, as well as localizedpoint concentration(s) such micro-pinholes and the like.

[0026] The rate of nanowire growth appears to be non-linear. The rate ofnanowire growth or emission is related to the relaxation of initialelevated compressive stress with the highest rates of nanowire growthoccurring during the initial phases of compressive stress relaxation.The emission of nanowire material can proceed for any desired interval.However, it is believed that the most productive or proficient intervalof nanowire formation occurs in the initial stages of compressive stressrelaxation. Production efficiency is believed to decrease as residualcompressive stress is relaxed.

[0027] It is believed that the production of nanowire is formed by aprocess of residual stress-induced extrusion on the microscopic scale.Extrusion typically occurs at intervals of discontinuity or fracture inthe thin film surface with the rate of growth decreasing over time.Greatest intervals of growth appear to occur within the first intervalof reduction of the first elevated compressive stress. The interval ofgreatest nanowire growth is in the first 24 to 48 hours with thegreatest productivity being in the first 24 hours.

[0028] The nanowire produced as a result of the process of the presentinvention is typically one which is composed of the metallic material ofthe first phase with little or no evidence of second phase matrixmaterial integrated therein. The nanowire is typically a wire formed ofsingle crystalline regions which can be hundreds of micrometers inlength. The single crystalline material can have any suitable structuresuch as the bulk rombohedral structure for bismuth nanowire. Thenanowire thus produced will have a diameter less than about 200nanometers, with diameters between 10 and 200 nanometers beingpreferred. The nanowire produced by the process of the present inventionhas an aspect ratio greater than about 100; with aspect ratios as greatas 100,000 being possible. It is believed that the diameter of theresulting nanowire is a function of the size of the initiation point inthe thin film.

[0029] In preparing the composite film of the present invention, it hasbeen found that metallic nanowires emanate or are emitted immediatelyupon completion of the codeposition film formation process from variousfractures or surface imperfections formed on the thin film.

[0030] The process of the present invention also contemplates a methodand materials for preventing immediate or spontaneous production ofnanowires in which a suitable cap or layer is placed in overlyingrelationship with the thin film immediately upon completion of thecodeposition film formation process. The cap or layer is one which iscapable of covering or blocking the fractures or surface imperfectionsites from which the nanowire emanates, thereby maintaining initialcompressive stress and preventing nanowire growth. The process of thepresent invention contemplates that the cap can be wholly or partiallyremoved to initiate nanowire emission. Thus, the cap layer will arrestgrowth and permit delayed release of metallic nanowire from theassociated matrix at a time desired.

[0031] Thus the present invention also contemplates a metallic nanowiregrowth structure 10 in which nanowire growth can be delayed and/orcontrolled. The delayed nanowire growth structure 10 comprises acomposite film 12 having at least a host matrix phase 14 and a diffusedmetallic phase 16 in which the host phase and metallic phase have amelting temperature differential of at least 30%. The composite film hasa compressive stress greater than 200 MPa and a sheath overlyingfracture or initiation points 20.

[0032] The sheath 18 or cap may be composed of any suitable materialwhich can overlay exposed surfaces of the composite film 12 andassociated fracture points 20 in a suitable adhering relationship toprevent extrusion of metallic material from fracture points 20. Asdepicted in FIG. 5, the sheath 18 is a layer overlaying the uppersurface 22 of composite film 12. The sheath 18 is a material which canadhere to the composite film 12 to prevent or, at minimum, retardextrusion of metallic nanowires and maintain the inherent compressivestress at or near the initial compressive stress preset thin filmformation. Materials which can be employed in the sheath 18 can includematerials such as non-reactive polymers, metals, ceramics, and the like.

[0033] As depicted in FIG. 2, the sheath 18 is partially removed fromcontact with the upper surface 24 of composite film 12. Such removalinitiates extrusion and growth of nanowires 22 from newly exposedfracture or stress concentration points 20.

[0034] In certain instances, it is also contemplated that nanowiregrowth can be initiated from the nanowire growth structure 10 of thepresent invention by scoring, scratching or piercing the sheath 18 in amanner which imparts or exploits underlying stress concentration pointsor regions in the composite film. This results in nanowires 22 emittingthrough associated orifices 26 produced in the sheath 18.

[0035] A portion of nanowire 22 is depicted in FIG. 3 emanating out ofassociated thin film 12 through a stress concentration point 20 such asa circular buckling crack, pinhole defect, or boundary between columnarfeatures as are common morphology of thin films formed by physical vapordeposition. Nanowire 22 can have extrusion marks such as ridges 28.Additionally nanowire 22 can have continuous bend contours 30 across thediameter of the nanowire which is indicative of the single crystalnature of the metallic nanowire 22.

[0036] To further describe the present invention, the following examplesare set forth. The examples are for illustrative purposes and are not tobe construed as limitations of the present invention.

EXAMPLE I

[0037] The various thin films of bismuth in a host matrix of chromenitride (CrN) were prepared in a commercial closed field unbalancedmagnetron sputtering system (Teer 550). Four sputter targets arranged ina rectangular geometry were used for depositions. Targets were of 99.99%pure Cr and one was of 99.9% pure Bi. Films were deposited on singlecrystal (Si)111 substrates. The substrates were cleaned ultrasonicallyin acetone and methanol before introduction into the vacuum chamber. Thebase pressure of the vacuum system was 6×10⁻⁶ torr. Immediately beforedeposition, the substrates were argon ion etched for about 30 minuteswith the substrates biased at −400V. An adhesion promotion layer of pureCr of approximately 100 to 200 nm thickness was deposited first in pureAr. The composite thin films were then deposited by rotating thesubstrates to sequentially face the four targets, using a sputtering gasmixture of 43% argon and 57% nitrogen. The purity of argon and nitrogenwas 99.999% and 99.99% respectively. The substrate bias voltage was −60Vfor all samples. It was applied during etching and deposition as voltagepulses of 500 nsec plus width and 250 kHz frequency. The substraterotation speed was such that about 10 nm thickness of material wasdeposited during each revolution. The total thicknesses of the filmsproduced ranged from 1.5 to 2.5 micrometers. The substrate temperaturewas less than 150° C.

[0038] Composition and purity of the films were determined by electronprobe microanalysis (EPMA) and Auger depth profiling. The bismuthconcentration was adjusted from 0.5 to 4.3% by varying the power on thebismuth target while maintaining constant power on the chrome target.X-ray diffraction (XRD) shows that the films consisted of a mixture ofelemental bismuth and chrome nitride (CrN). Thin films of CrN andelemental Bi were also made under similar sputtering conditions forpurity studies. The oxygen and carbon impurities in the CrN film werefound to be less than the detection limits of EPMA, which is about 0.1at. %. The total oxygen impurity in the bismuth film was 2 to 3 at. %,including surface oxides.

EXAMPLE II

[0039] Films made according to the process outlined in Example I wereinspected using scanning electron microscopy (SEM) imaging. An abundanceof wires was found on the surface of the composite films inspected asevidenced from the low magnification SEM image (see FIG. 4A) that givesa panoramic view of the sample surface. The wires were approximatelycircular in cross-section with diameters ranging from about 30 to 200nanometers. Most wires were very long compared to their diameter andcompared to the thickness of the thin film. A conservative estimate ofthe length of the wire loop shown in FIG. 4B suggests that it was atleast several millimeters in length. Various other samples wereinspected and such lengths were typically observed among the wires. Thehighest aspect ratio of the wires was calculated to be at least 100,000.

[0040] The thin films with associated wires were inspected to determinethe locations of wire emergence. Inspection indicated that wiresappeared to emerge from locations on the respective surfaces where pinhole defects existed. Other wires appeared to emerge from boundariesbetween columnar features that are a common morphology feature of thethin films formed by physical vapor deposition (see FIG. 4C).Occasionally, a circular buckling crack was observed around anassociated wire. The circular buckling crack indicated evidence of thehigh stress associated with the formation of the wires.

[0041] The thin film was analyzed for compressive stress. A compressivestress of about 470 MPa was observed in a thin film prepared accordingto the method of Example I deposited on a silicon substrate using thewafer curvature method and technique and Stoney's equation. The wireswere further inspected and showed evidence of extrusion marks. Theseobservations further supported the hypothesis that the wires were formedby residual stress-induced extrusion on the microscopic scale.

EXAMPLE III

[0042] To further demonstrate that bismuth nanowires were formed bystress, extrusion, films deposited on silicon substrates according tothe method of Example I were fractured and viewed in cross-section inSEM. FIGS. 5A and 5B present two SEM images of the same area of afractured sample taken 5 minutes apart. The SEM images clearly showedthat some of the wires grew from the freshly fractured surface duringSEM observation. A number of wires were observed to grow from the newlyfractured surface inside the SEM. Extremely high growth rates of about 5micrometers per second were observed. Nanowire growth rate in varioussamples was observed over time. It was found that the rate of nanowiregrowth decreased with time with the greatest rate of growth occurringduring the first 24 hours after film formation.

EXAMPLE IV

[0043] Nanowires produced from thin film composites prepared accordingto the method of Example I were analyzed to characterize nanowirestructure and composition. Various nanowires wires were further examinedusing TEM together with energy dispersive x-ray spectroscopy (EDS). TheEDS analysis detected bismuth with little or no chrome. This suggestedthat most of the bismuth wires were formed after completion of theco-deposition process. FIG. 6 is a TEM photo of several bismuth wiresprepared according to the process outlined in Example 1. The nanowiresshowed evidence of a series of continuous bend contours across theirrespective diameters when viewed in the TEM micrograph. This phenomenaindicated that the nanowires are composed of single crystals. Theelectron diffraction patterns showed that the structure of the nanowiresare bulk bismuth rombohedral structure. Selective area electrondiffraction of single nanowires also indicated that the wires weresingle crystals, though no predominate axis was identified. The singlecrystalline regions along the wires averaged several hundreds of micronsin length. Occasional grain boundaries were observed at kinks along thewires.

[0044] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures as is permitted under the law.

What is claimed is:
 1. A method for producing nanowires comprising thestep of: forming a composite film, such that the composite film of atleast two phases has at least one outwardly oriented surface, wherein afirst phase is a metallic material, the metallic material including atleast one of bismuth, indium, tin, lead, zinc, antimony, alloyscontaining bismuth, alloys containing indium, alloys containing tin,alloys containing lead, alloys containing zinc and alloys containingantimony, and a second phase functions as a host matrix, the compositefilm emitting nanowires composed of the metallic material of the firstphase and having an elongated crystalline structure, a diameter lessthan about 500 nanometers and an aspect ratio of at least
 100. 2. Themethod of claim 1 wherein the composite film has an initial highcompressive stress, the method further comprising the step of permittingthe initial high elevated stress to relax to a subsequent lowercompressive stress with associated diffusion of metallic material withinthe host matrix and emission from the outwardly oriented surface of thecomposite film..
 3. The method of claim 2 wherein the nanowire emitsfrom at least one initiation point.
 4. The method of claim 2 wherein therelaxation of compressive strength occurs at a controlled rate.
 5. Themethod of claim 1 further comprising the step of regulating thetemperature of the host matrix to adjust emission rate of the metallicmaterial.
 6. The method of claim 1 wherein the metallic material emitsfrom the location on the outwardly oriented film surface at a rate whichdecreases over time.
 7. The method of claim 1 wherein the host matrixhas a melting temperature at least 50% greater than the meltingtemperature of the metallic material.
 8. The method of claim 5 whereinthe host matrix has a melting temperature at least 30% greater than themelting temperature of the metallic material.
 9. The method of claim 1wherein the host matrix is composed of a composite which is essentiallynon-reactive with the metallic material.
 10. The method of claim 1wherein the host matrix is composed of at least one of nitride, carbide,oxide, and bromide.
 11. The method of claim 8 wherein the host matrixincludes a nitrogen containing at least one of chrome, titanium, andaluminum.
 12. The method of claim 1 wherein the host matrix is chromenitride.
 13. The method of claim 1 wherein the metallic material isemitted at a diameter less than about 200 nanometers.
 14. The method ofclaim 1 wherein the diameter is between 20 and 200 nanometers.
 15. Themethod of claim 1 wherein the emitted material has an aspect ratiobetween 100 and 100,000.
 16. The method of claim 1 wherein the elevatedcompressive stress of the thin film is sufficient to promote migrationof metallic material contained in the host matrix toward the at leastone emission location.
 17. The method of claim 15 wherein the elevatedcompressive stress is at least 200 MPa.
 18. The method of claim 16wherein the elevated compressive stress is at least 450 MPa.
 19. Themethod of claim 1 wherein the film has a thickness sufficient to permitdiffusion of the metallic material through the host matrix and emissionof at least a portion of the metallic material from the at least onelocation on the surface of the film.
 20. The method of claim 3 furthercomprising the step of producing at least one initiation point in thecomposite film.
 21. The method of claim 20 wherein the initiation pointformation occurs by at least one of scoring, poking or fracturing. 22.The method of claim 1 wherein the metallic material has a first meltingtemperature and the host matrix material has a second meltingtemperature at least 30% greater than the first melting temperature. 23.A delayed release metallic nanowire structure, the delayed releasenanowire structure comprising: a composite film having at least twophases wherein at least one phase contains a metallic compound and atleast one host matrix phase containing at least one of nitrides, carbon,oxides, borides, the composite film having an initial high elevatedcompressive stress, at least one initiation point, and at least oneoutwardly oriented surface; and a layer overlying the first surface ofthe composite film, the layer composed of at least one material which isessentially non-interactive with materials contained in the compositefilm, the layer being partially removable relative to the outwardlyoriented surface of the composite film.
 24. The delayed release metallicnanowire structure of claim 24 wherein the metallic compound is at leastone of bismuth, indium tin, lead, zinc, antimony, alloys containingbismuth, alloys containing indium, alloys containing tin, alloyscontaining lead, alloys containing zinc and alloys containing antimony.25. The delayed release metallic nanowire matrix of claim 23 wherein themetallonitrogen complex is chrome nitride.
 26. The delayed releasemetallic nanowire matrix of claim 23 wherein the overlying layer hassufficient thickness to isolate the host matrix from interaction withenvironment outside the matrix and to maintain the compressive stress ofthe host matrix at a level essentially approximate to the initialelevated compressive stress.
 27. The delayed release metallic nanowirestructure of claim 26 wherein the initial elevated compressive stress ofthe host matrix is sufficient to promote migration of the material inthe metallic phase to regions of stress concentration in the hostmatrix.
 28. The delayed release metallic nanowire structure of claim 23wherein the elevated compressive stress is at least 200 MPa.
 29. Ametallic nanowire composed of at least one of bismuth, indium, tin,lead, zinc, antimony, alloys containing bismuth, alloys containingindium, alloys containing tin, alloys containing lead, alloys containingzinc and alloys containing antimony, the metallic material present in atleast one single elongated crystal having an aspect ratio of at least100.
 30. The metallic nanowire of claim 29 wherein the material ofcomposition is at least one of bismuth, indium, tin, lead, zinc andantimony.
 31. The metallic nanowire of claim 30 wherein the aspect ratiois between 100 and 100,000 and the single crystalline region is at least50 microns.
 32. The metallic nanowire of claim 31 wherein the nanowirehas a diameter less than about 200 nanometers.
 33. The metallic nanowireof claim 31 wherein the diameter is greater than 20 nanometers.